1. Field of the Invention
This invention relates to an external cavity type light source used in coherent light communication and measurement technology fields.
2. Description of the Related Art
An external cavity type light source in a related art will be discussed with reference to FIGS. 4 and 5.
FIG. 4 is a block diagram to show an example of an external cavity type light source in a related art (Littrow optical system) and FIG. 5 is a graph to show the spectrum of output light of the external cavity type light source in FIG. 4.
In FIG. 4, numeral 1 denotes. a semiconductor laser, which will be hereinafter abbreviated to LD, numeral 2 denotes a diffraction grating, numerals 5, 6, and 7 denote lenses, numeral 8 denotes a light isolator, and numeral 10 denotes an optical fiber.
The LD 1 is coated on one end face la (end face on the diffraction rating 2 side) with a reflection prevention film 1A to prevent Fabry-Perot resonation on both end faces of the LD.
Emitted light from the end face 1a coated with the reflection prevention film 1A is converted into collimated light through the lens 6 and is incident on the diffraction grating 2. Of the light incident on the diffraction grating 2, only the light whose wavelength is selected through the diffraction grating 2 is diverted 180 degrees and advances, then is gathered through the lens 6 and is fed back into the LD 1. That is, an end face 1b of the LD 1 and the diffraction grating 2 make up an external resonator for lasing.
On the other hand, emitted light from the end face 1b of the LD 1 is converted into collimated light through the lens 5, passes through the light isolator 8, and is gathered through the lens 7, then is taken out as output light through the optical fiber 10.
However, in the external cavity type light source in the related art, as shown in FIG. 5, the output light contains naturally emitted light of a wide wavelength band emitted to the lens 5 directly from the LD 1 in addition to the laser light of a single wavelength selected through the diffraction grating 2, and light of a pure wavelength cannot be taken out as output light.
Specifically, in the example of the light source in the related art, the side mode suppression ratio (power ratio between the laser light of a single wavelength and the naturally emitted light of a wide wavelength band) is about 40 to 50 dB.
The external cavity type light source based on a general Littrow optical system as described above involves the following problem: In wavelength sweeping, mode hop occurs and light output varies.
It is therefore an object of the invention to provide an external cavity type light source which enables light of a pure wavelength with extremely less naturally emitted light to be taken out as output light and can prevent mode hop from occurring for enabling continuous wavelength sweeping with less variations in light output.
To achieve the above object, according to a first aspect of the invention, there is provided an external cavity type light source comprising a semiconductor laser coated on one end with a reflection prevention film, wherein emitted light from the end face of the semiconductor laser on the reflection prevention film side is converted into collimated light and the collimated light having a wavelength selected through a diffraction grating is fed back into the semiconductor laser, and wherein emitted light from an opposite end of the semiconductor laser is converted into collimated light and the collimated light is gathered and output to an optical fiber, characterized in that a light branch element is placed between the diffraction grating and the semiconductor laser, and that diffracted light fed back into the semiconductor laser from the diffraction grating is made to branch through the light branch element and one branch light is taken out as output light.
According to the first aspect of the invention, a light branch element is placed between the diffraction grating and the semiconductor laser, and the diffracted light fed back into the semiconductor laser from the diffraction grating is made to branch through the light branch element and one branch light is taken out as output light, so that it is made possible to decrease the naturally emitted light component from the semiconductor laser, contained in the output light. That is, it is made possible to take out light of a pure wavelength with extremely less naturally emitted light as output light.
In a second aspect of the invention, in the external cavity type light source as set forth in the first aspect of the invention, diffracted light having a wavelength selected through the diffraction grating is once applied vertically to a mirror and is again returned to the diffraction grating, then is fed back into the semiconductor laser.
According to the second aspect of the the invention, the diffracted light having a wavelength selected through the diffraction grating is once applied vertically to the mirror and is again returned to the diffraction grating, then is fed back into the semiconductor laser, thus the wavelength selectivity is furthermore enhanced.
In a third aspect of the invention, the external cavity type light source as set forth in the second aspect of the invention further includes a turning mechanism capable of turning the mirror.
According to the third aspect of the invention, the turning mechanism capable of turning the mirror is provided, so that wavelength sweeping corresponding to the turning angle of the mirror is enabled.
In a fourth aspect of the invention, in the external cavity type light source as set forth in the third asepectof the invention, the intersection point of a line extended vertically to an optical axis with an optical position of the end face of the semiconductor laser on the side coated with no reflection prevention film with respect to the diffraction grating as a starting point and an extensions of a diffraction face of the diffraction grating is matched with the turning center of the mirror and the mirror is placed so that the extension of a reflection face of the mirror passes through the intersection point.
According to the fourth aspect of the invention, the intersection point of the line extended vertically to the optical axis with the optical position of the end face of the semiconductor laser on the side coated with no reflection prevention film with respect to the diffraction grating as the starting point and the extension of the diffraction face of the diffraction grating is matched with the turning center of the mirror, and the mirror is placed so that the extension of the reflection face of the mirror passes through the intersection point, so that occurrence of mode hop can be prevented over a wide range and continuous wavelength sweeping with less variations in light output is enabled.
In a fifth aspect of the invention, in the external cavity type light source as claimed in any of the first to fourth aspects of the invention, the light branch element is formed of an unpolarized beam splitter.
According to the fifth aspect of the invention, the diffracted light fed back into the semiconductor laser from the diffraction grating is made to branch through the unpolarized beam splitter and one branch light is taken out as output light.